Projection optical systems present many optical design challenges, several of which require a tradeoff between conflicting design goals. Foremost is the requirement of producing a high-quality image, characterized by low distortion, high resolution, high contrast ratio, high light throughput, and uniform screen illumination. In addition to the image quality requirement, the operational environment for many optical projectors introduces further constraints. For example, many projectors must project images large enough to be seen by many people, yet fit into a rather small conference room. This constraint requires the projection system to have a relatively wide field angle so that a large image may be produced with a short projection distance. Other lenses, such as those described herein which are used in large venue applications, do not require the wide field angles, but instead require excellent lateral color correction.
Additionally, the projection systems are rarely positioned level with the center of the projection screen. Because the projector system and the center of the screen are not located at the same elevation, the image may exhibit a keystoning effect. Keystoning occurs when an image is projected at an angle onto a screen causing the side of the image furthest from the projector to be wider than the side of the image closest to the projector. Keystoning often occurs when a table-top overhead projector projects an image upward onto a wall screen. Because the top of a projected square object will be wider than the bottom, the object will resemble the wedge-shaped piece at the crown of a stone archway called a keystone. Wide field angles exacerbate the problem of keystoning.
Existing lenses that have wide field angles, low distortion, high resolution, high contrast ratio, high light throughput, and uniform screen illumination are either too small format, have too short of a back focal distance, or have insufficient lateral color correction. Therefore, there is a need in the art for a high resolution projection lens having a long back focal distance, a wide field angle, high contrast ratio, low distortion, high light throughput, and uniform screen illumination.